J.C. Bourgoin

Mechanism of GaAs transport by water reaction application to the growth of thick epitaxial layers

We study in detail the decomposition of GaAs by water, and show that the growth technique of epitaxial layers based on this reaction can lead to growth rates reaching several μm per minute. This opens an economical access to the production of semi-insulating and thick epitaxial layers. We briefly mention that these layers can exhibit electronic properties allowing their use in various fields such as high power electronics, photodetection and micro-electronics (production of very homogeneous semi-insulating layers).

A comparison between GaAs and CdTe for X-ray imaging

We have grown 4 inch GaAs epitaxial layers of thickness ranging from 100 to 600 /spl mu/m. With such layers we made pixel X-ray detectors where each pixel is a p/sup +//i/n/sup +/ mesa structure in which the grown layer is the i region. The aim of this communication is to describe, prior to the evaluation of an image, the performances of such a detector in terms of homogeneity, reverse dark current, linearity, response time, dynamic range and charge collection efficiency. We shall also describe how the current induced by 100 kV X-rays in a detector made of a 115-/spl mu/m thick GaAs layer compares with that produced in the same conditions by a 1-mm thick CdTe detector.

Characterization of thick epitaxial GaAs layers for X-ray detection

Abstract We have studied the current–voltage and capacitance–voltage characteristics of p/i/n structures made on non-intentionally doped epitaxial GaAs layers grown by the chemical reaction method. Deep level transient spectroscopy demonstrates that these layers contain a low defect concentration. X-ray photoconductivity shows that the diffusion length is large. The homogeneity of the properties of these layers, which has been evaluated over large area (cm 2 ), is confirmed by photoluminescence mapping.

Performances of epitaxial GaAs detectors

The aim of the study is to show that the thick epitaxial layers we grow using the Chemical Reaction method, a method which overcomes the difficulties in producing thick epitaxial layers, can provide detectors which have the same high performances, established since the 1970s and recently confirmed, as the ones made by conventional epitaxial methods. For this, we performed counting detection of high-energy protons, electrons, gamma and alpha particles with detectors made with such layers. The results obtained in each case are discussed in terms of energy resolution and charge collection efficiency.

Performances of epitaxial GaAs p/i/n structures for X-ray imaging

We have realized 150mm � 150mm pixels using ion implantation followed by photolithography, metallic contact evaporation and chemical etching on about 200mm thick GaAs epitaxial layers. These layers were grown on n + and p + substrates by an already described Chemical Reaction technique, which is economical, non-polluting and can attain growth rates ofseveral microns per minute. The mesa p + /i/n + pixel were characterized using current–voltage and capacitance–voltage measurements. The charge collection efficiency was evaluated by photoconductivity measurements under typical conditions ofstandard radiological examinations. r 2002 Elsevier Science B.V. All rights reserved.

X-ray detector with thick epitaxial GaAs grown by chemical reaction

Thick (200 to 500 /spl mu/m) epitaxial GaAs layers have been grown on two inches wafers by using a chemical reaction technique introduced recently which is cheap, non-polluting and allows to reach very high growth rates. X-ray detectors made of p/i/n structures have been realized using non-intentionally doped layers grown on n/sup +/ GaAs substrates, with the p/sup +/ layer at the surface obtained by ion implantation. These detectors have been validated by current and capacitance-voltage measurements, photocurrent induced by X-ray irradiation, and energy resolution measurements. The data obtained demonstrate that these detectors exhibit similar performances as those obtained previously with conventional epigrowth techniques. Under standard conditions of medical examination (anode voltage of 60 kV, current of 75 mA and distance of 70 cm), Up to 10/sup 13/ charges per second per mm/sup 2/ can be collected. The observed response time is apparently limited by the decay of the X-ray pulse.

A method for adjusting the performances of epitaxial GaAs X-ray detectors

To detect high-energy photons using compound semiconductor detectors such as GaAs requires enlargement of the depleted zone, which is limited by the residual doping of the semiconductor. We discuss a technique by which the extension of the space charge region of a diode can be increased. It consists in compensating the residual doping impurities with defects introduced by electron irradiation. Results are presented to illustrate and evaluate the limits of this technique in the case of GaAs p/i/n structures.

Laser-induced pulse shapes in partially depleted epitaxial GaAs radiation detectors

AbstractMeasurements of laser-induced pulse shapes are reported in partially-depleted 150mm thick epitaxial GaAs, carriedout as a function of device temperature and bias voltage. Residual impurity concentrations of 1–4 10 14 cm 3 at roomtemperature drop to 7 1310 cm 3 at 219K, resulting in a significant increase in the depth of the space charge region. Afocussed pulsed laser beam was used to probe the cleaved edge of a device and so investigate the dependence of thesignalpulse shape onthe interactiondistance from the Schottkycontact. Carrier lifetimes ofup to500mswere observedconsistent with charge diffusion in the low-field region of the device. Photo-induced current transient spectroscopymeasurementsprovidefurtherevidenceofanumberofmidbandgaptrapswithpeakemissionratesbetween300–330K.r 2003 Elsevier B.V. All rights reserved. PACS: 29.40.Wk; 73.20.JvKeywords: Epitaxial gallium arsenide; Radiation detector; Laser-induced pulse shape 1. IntroductionThis paper reports the ongoing characterisationof thick epitaxial GaAs layers produced by achemical reaction technique [1,2], which havepotential uses in X-ray imaging radiation detec-tors. Thick epitaxially grown GaAs generallyshows excellent whole-wafer uniformity and lowEL2 concentration and therefore is well suited forthe fabrication of X-ray imaging sensors [3].However, the material suffers from a shallowdepletion thickness due to impurities that areintroduced during growth. In this work we haveinvestigated the effect on the carrier concentrationand depletion thickness of cooling the devices inthe range 200–300K. A focussed pulsed laser isscanned across the cleaved edge of the detectorfrom cathode to anode to replicate radiationinteractions in the devices. Analysis of the result-ing pulse shapes allows the investigation ofchanges in depletion thickness as a function oftemperature and bias voltage.2. ExperimentalmethodTest detectors were fabricated from epitaxialGaAs material grown by the chemical reactionmethod. The lightly n-type epitaxial GaAs was150mm thick, grown onto a low resistivity n-type

Application of epitaxial GaAs to medical imaging

We show that GaAs is a promising material for X-ray medical imaging detectors. Indeed, fluorescence degrades the spatial resolution, and GaAs optimises the compromise between absorption and spatial resolution. We describe the performances (spatial resolution, linearity, energy resolution and temporal response) reached by detectors made of the thick epitaxial GaAs layers we produce, in order to illustrate that such detectors qualify for medical imaging. Finally the main limitation, related to the level of residual doping in an epitaxial layer, is discussed.

Charge collection in epitaxial GaAs p/i/n radiation detectors

We describe the mode of operation of a detector for direct photon-electron conversion at room temperature, made of epitaxially grown GaAs. Contrary to bulk grown materials, epitaxial layers are free of defects, i.e. exhibit long lifetimes and high carrier mobilities, and have uniform electronic properties. However, the depleted zone is of limited extension, consequence of the level of the residual doping impurities, which are not compensated by defects. These detectors are adapted to X-ray imaging, in particular for low energy medical applications such as mammography, because of the availability of large areas (up to 4 inches in diameter), standard technological processes for making pixellated detectors and cost. However, charges in the neutral region can be collected by diffusion and we shall present data allowing to illustrate and evaluate this effect. Finally photocurrent measurements obtained under medical conditions demonstrate that, for the detector used, only a small fraction of the photocurrent originates from diffusing charges. They also show how a 120 μm thick GaAs epitaxial detector competes with a 0.5 mm thick CdZnTe detector.